You may be correct as I think I saw an informal survey of iPhone 4S buyers that stated only a small percentage were upgrading.
However, iPhone 3GS users for the most part have had the phone 24 to 12 months, and if you are eligible for an upgrade with AT&T, it seems you pay the iphone subsidy each month (by my estimates, around $50 per month) regardless of whether you upgrade or not. So it makes sense to upgrade and sell the new phone on ebay if you want to keep your 3GS.